Series-sequenced gaseous discharge devices, such as fluorescent lamps, are usually coupled to ballast apparatus which includes a transformer coupled to a potential source and to the ends of the series-sequenced gaseous discharge devices. A filament from each of the discharge devices is parallel-connected and these parallel connected filaments are coupled to a transformer which provides the energy necessary to operate the filaments. Usually, a starting capacitor shunts one of the series-sequenced discharge devices to provide an increased potential for initiating conduction in the un-shunted discharge device and, subsequently, in the shunted discharge device.
Although the above-described series-sequenced ballast apparatus and gaseous discharge devices utilizing a parallel-connected filament arrangement have been and still are employed in numerous applications, it has been found that such a circuit configuration does leave something to be desired. More specifically, series-sequenced discharge devices employing parallel-connected filaments tend to require relatively complex and expensive wiring arrangements. Moreover, energy losses in ballast apparatus designed for series-sequenced lamps with parallel-connected filaments tend to exceed the energy losses of series-sequenced lamps wherein the filaments are series-connected.
However, even though the above-mentioned advantages of series-connected filaments for series-sequenced discharge devices have been known for some time, large scale implementation of series-connected discharge devices has been delayed due to a serious impairment of lamp performance with such a series-connected circuit configuration. More specifically, it has been found that removal or failure of one of the discharge devices results in an undesirable condition wherein the remaining gaseous discharge device operates in a so called cold cathode glow discharge condition.
As an example, FIG. 1 illustrates a prior art series-sequenced series-connected configuration. Herein, a transformer 5 is coupled to the opposite ends of a pair of series-sequenced gaseous discharge devices 7 and 9 respectively. A capacitor 11 shunts one of the discharge devices 7, and a circuit means 13 series-connects a filament of each of the discharge devices 7 and 9 to the transformer 5.
In operation, failure of the discharge device 7, specifically the series-connected filament of discharge device 7, would interrupt the series-connected circuit means 13 and inhibit heating energy to the filament of the discharge device 9. However, current would undesirably flow by way of the transformer 5, shunting capacitor 11 and the circuit means 13 to the other discharge device 9. Thus, the other gaseous discharge device would operate in a cold cathode glow discharge mode which is both undesirable and deleterious to extended utilization of the other gaseous discharge device 9, to wit, failure of device 9.